U.S. patent application number 12/262591 was filed with the patent office on 2009-02-26 for system and method for optical transmission.
This patent application is currently assigned to Cisco Technology, Inc.. Invention is credited to BRYANT A. BEST.
Application Number | 20090052908 12/262591 |
Document ID | / |
Family ID | 35429662 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090052908 |
Kind Code |
A1 |
BEST; BRYANT A. |
February 26, 2009 |
SYSTEM AND METHOD FOR OPTICAL TRANSMISSION
Abstract
In an example embodiment, an optical system includes an analog
laser transmitter having a burst operative mode. The system may
include enable logic configured to provide a digital enable signal
in response to receiving a trigger signal other than a reverse
electrical signal. The system may further include a power
controller circuit coupled to the transmitter and enable logic and
configured to turn on the analog transmitter when the enable signal
is asserted and turn off the analog transmitter when the enable
signal is de-asserted. An example method embodiment may include
providing an analog signal to the analog laser transmitter and
providing a trigger signal to the enable logic to provide a digital
enable signal in response to receiving the trigger signal and
turning on the analog laser transmitter when the enable signal is
asserted, and turning off the analog laser transmitter when the
enable signal is de-asserted.
Inventors: |
BEST; BRYANT A.; (Flowery
Branch, GA) |
Correspondence
Address: |
SCIENTIFIC-ATLANTA, INC.;INTELLECTUAL PROPERTY DEPARTMENT
5030 SUGARLOAF PARKWAY
LAWRENCEVILLE
GA
30044
US
|
Assignee: |
Cisco Technology, Inc.
San Jose
CA
|
Family ID: |
35429662 |
Appl. No.: |
12/262591 |
Filed: |
October 31, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10930214 |
Aug 31, 2004 |
|
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12262591 |
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Current U.S.
Class: |
398/182 |
Current CPC
Class: |
H04B 10/564
20130101 |
Class at
Publication: |
398/182 |
International
Class: |
H04B 10/04 20060101
H04B010/04 |
Claims
1. An optical system comprising: an analog transmitter configured
for burst mode operation; enable logic configured to provide a
digital enable signal in response to receiving a trigger signal; a
power controller circuit coupled to said transmitter and said
enable logic, said power controller circuit configured to turn on
said analog transmitter when said enable signal is asserted and
turn off said analog transmitter when said enable signal is
de-asserted; and wherein said trigger signal comprises other than a
reverse electrical signal.
2. The optical system of claim 1, wherein said enable logic is
configured to receive a plurality of parallel trigger signals.
3. The optical system of claim 1, wherein said power controller
circuit is configured to control said transmitter output power
generated while said transmitter is turned on.
4. The optical system of claim 3, wherein said power controller
uses a monitor current to control said transmitter output power
generated while said transmitter is turned on.
5. The optical system of claim 1, wherein said trigger signal
comprises a control signal.
6. The optical system of claim 5, wherein said trigger signal
comprises a control signal originating at a cable modem terminating
system (CMTS).
7. The optical system of claim 5, wherein said trigger signal
comprises a time-division-multiple-access (TDMA) control
signal.
8. The optical system of claim 1, wherein said trigger signal is
not a reverse electrical signal.
9. A method of operating an analog laser transmitter, the method
comprising: providing an analog input signal to the analog laser
transmitter; providing a trigger signal to enable logic configured
to provide a digital enable signal in response to receiving said
trigger signal, said trigger signal comprising other than a reverse
electrical signal; providing said digital enable signal to a
controller circuit coupled to both said enable logic and to said
analog transmitter; and said controller circuit turning on said
analog laser transmitter when said enable signal is asserted, and
turning off said analog laser transmitter when said enable signal
is de-asserted.
10. The method of claim 9, wherein said trigger signal comprises a
control signal.
11. The method of claim 10, wherein said control signal originates
at a cable modem terminating system (CMTS).
12. The method of claim 10, wherein said control signal comprises a
time-division-multiple-access (TDMA) control signal.
13. The method of claim 9, wherein said trigger signal comprises a
"signal valid" signal from a carrier detect circuit.
14. The method of claim 9, wherein said trigger signal is not a
reverse electrical signal.
15. The method of claim 9, wherein said enable signal comprises
said trigger signal.
16. The method of claim 9, further comprising said power controller
circuit controlling said transmitter output power generated while
said transmitter is turned on.
17. The method of claim 16, wherein said controlling said
transmitter output power comprises utilizing a laser monitor
current in a feedback loop.
18. The method of claim 9, wherein said providing a trigger signal
comprises providing a plurality of parallel trigger signals.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
10/930,214, filed Aug. 31, 2004.
TECHNICAL FIELD
[0002] The present disclosure is generally related to optical
systems, and, more particularly, is related to laser
transmitters.
BACKGROUND OF THE DISCLOSURE
[0003] Optical transmission systems incorporating laser
transmitters can be generally classified under two broad
categories: analog systems and digital systems. In analog systems,
the laser transmitter operates in an analog mode that can be
described using the prior art optical system shown in FIG. 1. Laser
transmitter 120 accepts an analog input signal via electrical line
110 and produces an output optical signal on optical link 140. In
certain applications, the analog input signal may be a baseband
analog signal such as a sine wave of a particular frequency, or a
complex analog signal containing several frequency components. In
other applications the analog input signal may be a modulated
signal containing carrier frequency components. Typically, laser
transmitter 120 is a broadband device that can accommodate the
range of frequencies present in the input signal, and carries out
an electrical-to-optical conversion process by converting the
electrical-domain analog input signal into an optical-domain output
optical signal.
[0004] Power controller 130 operates together with laser
transmitter 120 to produce the output optical signal at a desired
power level while simultaneously ensuring that laser transmitter
120 operates in a linear operative mode that produces certain
desirable characteristics. Such desirable characteristics include
output power accuracy, stability over time and environmental
conditions, flat frequency response, and low harmonic distortion.
Consequently, power controller 130 typically incorporates several
temperature-stable components as well as feedback circuitry. The
bandwidth and response time of the feedback circuitry is typically
chosen to filter out unwanted high-frequency perturbations and to
provide the desired characteristics mentioned above. As a
consequence, the turn-on/turn-off times of the laser transmitter
120 is fairly long. While this may be acceptable for the analog
system of FIG. 1, such lengthy turn-on/turn-off times prove
unsuitable for digital mode operations wherein the laser
transmitter has to be turned on and off more rapidly.
[0005] FIG. 2 illustrates a prior art optical system operating in a
digital mode. Laser transmitter 220 accepts a digital input signal
via electrical line 210 and produces an output optical signal on
optical link 240. The digital input signal is typically a baseband
signal that uses two or more logic levels. When two levels are
used, the system is termed a binary system, and the laser
transmitter operates in two distinct modes--either on or off. The
on state corresponds to a first logic level, while the off state
corresponds to a second logic level. The speed at which the laser
transmitter 220 can change from one state to the other (e.g. from
on to off) determines the maximum digital signal rate that can be
accommodated. The degree of illumination or lack thereof in the
output of the laser transmitter 220, often referred to as the
extinction ratio, determines the error rate encountered in decoding
the optical digital signal at a receiver located at the other end
of optical link 240. For example, if the laser transmitter 220 did
not turn off completely, an optical receiver at a distant end may
erroneously interpret the digital logic present in optical link 240
at that instant as a logic "one" rather than a logic "zero."
Consequently, laser transmitter 220 operates in a saturated mode of
operation wherein a laser inside laser transmitter 220 is
completely on or completely off. This is in contrast to the analog
system of FIG. 1 where a laser inside laser transmitter 120
operates in a linear manner and produces various non-discrete
levels of intensities.
[0006] Power controller 230 is designed to place laser transmitter
220 in the digital mode of operation where characteristics such as
extinction ratio, rise and fall times, and bit rate are the primary
criteria in contrast to characteristics such as flat frequency
response and low harmonic distortion that are desirable in the
system of FIG. 1. It can therefore, be appreciated that the two
prior art systems illustrated in FIGS. 1 and 2, prove to have
certain handicaps when a mixed mode of operation (digital and
analog) is desired. Thus, a heretofore unaddressed need exists in
the industry to address the aforementioned and/or other
deficiencies and inadequacies.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The preferred embodiments of the disclosure can be better
understood with reference to the following drawings. The components
in the drawings are not necessarily to scale, emphasis instead
being placed upon clearly illustrating the principles of the
present disclosure. Moreover, in the drawings, like reference
numerals designate corresponding parts throughout the several
views.
[0008] FIG. 1 illustrates a prior art optical system operating in
an analog mode.
[0009] FIG. 2 illustrates a prior art optical system operating in a
digital mode.
[0010] FIG. 3 illustrates an exemplary optical system that is a
passive optical network (PON) system incorporating at least one
modem located inside at least one residence.
[0011] FIG. 4 is a block diagram of a laser transmitter and power
controller that are located inside the modem of FIG. 3.
[0012] FIG. 5 shows an enable logic block associated with the laser
transmitter and power controller blocks of FIG. 4.
[0013] FIG. 6 illustrates component details of the circuitry
contained in the power controller and laser transmitter blocks of
FIG. 4.
[0014] FIG. 7 is a flow chart of a method of operating an analog
laser transmitter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] While the description below refers to certain exemplary
embodiments, it is to be understood that the disclosure is not
limited to these particular embodiments. On the contrary, the
intent is to cover all alternatives, modifications and equivalents
included within the spirit and scope of the disclosure. Also, the
terminology used herein is for the purpose of description and not
of limitation.
[0016] Attention is drawn to FIG. 3, which illustrates an exemplary
optical system 350 that is a passive optical network (PON) system
incorporating at least one modem 310 located inside at least one
residence 301. It will be understood that the modems 310, 320, and
330 shown in FIG. 3A are shown located inside their respective
residences merely for purposes of illustrating one exemplary
embodiment. In other alternative embodiments, one or more of these
modems are installed external to a residence, for example inside a
network interface device (NID). When installed external to the
residence, the internal wiring of the residence is left
undisturbed.
[0017] While PON systems are used in several types of communication
networks, the system shown in FIG. 3 as an example system, is a
part of a cable network. A few examples of communication networks
that may utilize PON systems, include fiber-to-the-home (FTTH),
fiber-to-the-curb (FTTC), and synchronous optical network (SONET)
networks.
[0018] The major functional blocks of the optical system 350
include, in addition to the cable modems, a cable modem terminating
system (CMTS) 370 and a PON combiner/splitter 360. System 350 can
operate as a bi-directional optical system to provide multimedia
cable services to multiple residences. A typical PON system may be
designed to serve 32 residences, of which three (301, 302, and 304)
are shown in FIG. 3.
[0019] Multimedia cable services include uni-directional services
such as for example, video delivery of television and movies, as
well as bi-directional services such as for example,
Internet-related data. Typically, the data-flow, whether
uni-directional or bi-directional, from CMTS 370 towards a
residence is referred to as a downstream delivery, while data-flow
from a residence towards the CMTS 370 is referred to as an upstream
delivery. In the downstream direction, CMTS 370 originates an
optical signal carrying the multi-media cable service. This optical
signal is transported via optical link 371 to PON combiner/splitter
360. In the downstream direction, PON combiner/splitter 360
operates as a splitter, and distributes the downstream optical
signal to multiple optical links such as links 361, 362, and 363.
Among several transport formats that can be employed to transport
the downstream signal, several communication systems use the
time-division-multiplexing (TDM) format to combine multiple pieces
of data, where the individual pieces are destined to individual
residences. TDM is a well-known format, and will not be elaborated
here, as persons of ordinary skill in the art will recognize this
technology.
[0020] In the upstream direction, a modem, such as modem 310,
originates data that is transported via optical links, such as link
361, to PON combiner/splitter 360. In the upstream direction, PON
combiner/splitter 360 operates as a combiner, and combines multiple
upstream optical signals from multiple optical links such as links
361, 362, and 363. The combined optical signal is then transported
via link 371 from PON combiner/splitter 360 to CMTS 370. It can be
understood that in combining several optical signals from multiple
optical links, the PON combiner/splitter 360 may encounter data
collisions that can corrupt the upstream data towards the CMTS 370.
Such undesirable data corruption can occur if two modems are
transmitting simultaneously at any given instance in time.
Consequently, a transport technology such as
time-division-multiple-access (TDMA) may be employed to avoid such
corruption. TDMA is one of several transport mechanisms, others,
for example, being wave-division-multiple-access (WDMA),
code-division-multiple-access (CDMA), and
subcarrier-multiple-access (SCMA).
[0021] In TDMA, each of the modems (e.g. modems 310, 320, and 330)
are synchronized in time with each other. Typically, this is
carried out using various methods, including "ranging" where the
CMTS 370, or another upstream device, determines how far away in
distance each modem is located, and assigns an optimal transmission
time slot to each modem to avoid transmission interference with
other modems. Various mechanisms are employed to carry out the time
slot assignment. As one example among many, message-based signaling
may be employed to convey the time slot information to the modems.
Furthermore, various mechanisms are employed to ensure that no two
TDMA time slots overlap one another as such overlaps can lead to
data collisions. One such mechanism, employs guard bands that are
periods between time slots where none of the modems perform
upstream transmission. Naturally, the presence of guard bands is
wasteful as they reduce the available data bandwidth for upstream
transmission.
[0022] When no guard bands are employed, a first laser transmitter
located in a first modem may not be completely turned off while a
second laser transmitter in a second modem begins to emit light,
thereby leading to the two light beams colliding with each other
and leading to data loss or corruption. It can be understood that
the laser transmitters should be turned on or turned off in a
minimal amount of time to avoid wasting upstream bandwidth. It can
also be understood that optical components of an optical system are
typically expensive, and consequently, there is a need to provide
systems that are cost-effective. The modems, such as for example,
modem 310, include an upstream transmitter 300, which incorporates
a laser transmitter and associated circuitry.
[0023] Attention is now drawn to FIG. 4, which illustrates a few
functional blocks of one embodiment of an upstream transmitter 300
of optical system 350. In one exemplary embodiment, among many,
upstream transmitter 300 is an analog-modulated cable TV (CATV)
reverse transmitter operating in the range of 5 to 42 MHz
(approximately), transmitting an optical signal of approximately 2
dBm power, the transmitter being designed to operate over a
temperature range of -40.degree. C. to +85.degree. C.
[0024] Laser transmitter 420 receives an electrical signal via line
410 and transmits an optical signal into optical link 361, which
transports the signal into PON combiner/splitter 360 of FIG. 3. The
electrical signal on line 410 comprises analog as well as digital
signals. In a first embodiment, among many, the electrical signal
is an analog baseband signal. In a second embodiment, the
electrical signal is an analog wideband signal. In a third
embodiment, the electrical signal is a modulated analog signal
comprising a radio-frequency (RF) carrier signal. In a fourth
embodiment, the electrical signal is a modulated digital
signal.
[0025] FIG. 4 depicts one example of an electrical signal. In this
example, the electrical signal is a m-ary modulated signal 401.
Where m=4, the m-ary signal is a quadrature amplitude modulated
(QAM) signal. Where m=64, the m-ary signal is referred to as a 64
QAM signal.
[0026] In a preferred embodiment of the disclosure, the electrical
signal on line 410 comprises an upstream signal conforming to the
Data Over Cable Service Interface Specification (DOCSIS). The
DOCSIS specification, inclusive of its various versions, is
incorporated herein by reference in its entirety. Persons of
ordinary skill in the art will recognize that the upstream signal
can employ various modulation schemes such as but not limited to,
quadrature phase shift keying (QPSK), 64 QAM, and 128 QAM; and
operate at various frequency ranges, such as, but not limited to, a
range of 5 to 42 MHz. Furthermore, the upstream signal can operate
at various data rates, for example, at 5.12 Msym/sec for a channel
width of 6.4 MHz.
[0027] Laser transmitter 420 comprises a laser diode and a monitor
diode that are components of an analog transmitter for transmitting
analog signals. Typically, a laser transmitter such as laser
transmitter 420 is a device designed for use in analog systems that
operate the laser diode in a linear operative mode. In the present
disclosure, laser transmitter 420 is configured to operate in a
burst operative mode, wherein when the laser diode is turned on,
the output optical signal on link 361 quickly reaches its desired
operating power with low overshoot characteristics. In one
exemplary embodiment, the optical signal reaches its operating
power within approximately one microsecond, together with a
transient response having less than substantially 5% overshoot.
Similarly, when the laser diode is turned off, the optical signal
on link 361 decreases quickly to zero with little undershoot. Such
a configuration wherein the analog laser transmitter 420 is
switched in a binary (on-off) mode is referred to as a burst
operative mode.
[0028] Power controller 430 interacts with laser transmitter 420 to
place the laser transmitter in the burst operative mode. Line 421
carries a monitor current produced in the monitor diode inside
laser transmitter 420, while line 422 carries the current flowing
through the laser diode inside laser transmitter 420. Power
controller 430 uses the monitor current of line 421 to control the
amplitude of the current flowing in line 422. Consequently, power
controller 430 can configure a desired optical power output of the
optical signal on link 361, and also control the presence/absence
of this output signal (burst operative mode). In this embodiment,
power controller receives an enable signal via line 450. When the
enable signal is asserted, power controller 430 turns on the laser
diode inside laser transmitter 420, and, alternatively, when the
enable signal is not asserted, power controller 430 turns off the
laser diode inside laser transmitter 420. This operation is shown
in FIG. 4, wherein the output optical signal 402 is present between
times t.sub.1 and t.sub.2 when the enable signal is asserted, and
wherein the output optical signal is absent when the enable signal
is not asserted as between times 0 and t.sub.1.
[0029] Attention is now drawn to FIG. 5, which shows an additional
block--enable logic 510. Enable logic 510 generates the enable
signal carried on line 450 to power controller 430. A clock signal
provided on line 511 ensures synchronous operation as is desirable
in a TDMA implementation. The enable signal is generated using one
or more trigger signals that are provided to enable logic 510. It
will be understood that one or more of the trigger signals
illustrated in FIG. 5 may be used in alternative embodiments.
[0030] Consequently, in a first embodiment only one trigger signal
is connected to enable logic 510, while in other embodiments more
than one trigger signal may be provided to enable logic 510. When
more than one trigger signal is connected to enable logic 510, in a
first embodiment, enable logic 510 selects one among the multiple
trigger signal inputs, while in a second embodiment, enable logic
510 selects a combination of trigger signal inputs.
[0031] A few examples, among many of trigger signals, includes a
control signal from CMTS 370. In one embodiment, the control signal
that is carried on line 512, comprises a message that was embedded
inside a downstream signal originated by CMTS 370, while in a
second embodiment, the control signal comprises a logic signal that
is asserted to indicate a trigger for generating the enable signal.
Persons of ordinary skill in the art will understand that using a
logic signal incorporates the use of logic levels and transition
edges.
[0032] In an alternative embodiment, a trigger signal comprises a
"signal valid" signal carried on line 513. One example of a circuit
that originates the "signal valid" signal, is a carrier detect
circuit 510 that is described in U.S. Pat. No. 6,509,994 B2, which
is herein incorporated by reference. The carrier detect circuit is
used to selectively activate the laser transmitter 420 through the
power controller 430 only when such activation is necessary. Such a
mode of activation allows power conservation, among other
benefits.
[0033] In yet another alternative embodiment, trigger signal
comprises a TDMA control signal carried on line 514. The TDMA
control signal is generated by other circuitry (not shown) of
upstream transmitter 300. Such a control signal can be generated in
response to a message from CMTS 370, or can be locally generated in
the upstream transmitter 300 using other means to synchronize with
one or more modems of the optical system 350.
[0034] A fourth example of a trigger signal is shown on line 516.
This trigger signal is derived from one or more synchronous
sources, such as multiple modems or a central control processor
located in the upstream transmitter 300.
[0035] Turning now to FIG. 6, which illustrates component details
of the circuitry contained in one example of the power controller
and laser transmitter blocks of earlier figures, attention is drawn
to laser transmitter 420. Laser diode 601 is a laser diode that is
optically coupled into optical link 361. Details of the optical
coupling will be omitted in this disclosure, because persons of
ordinary skill in the art will recognize the procedure to do so.
Monitor diode 602, which is optically coupled to laser diode 601,
provides a laser monitor current that is directly proportional to
the current flowing through laser diode 601.
[0036] In general terms, FIG. 6 comprises a system wherein the
laser monitor current is utilized in a feedback loop 652 to control
the laser diode current and consequently, the optical power
generated by the laser diode. The laser diode current is typically
preset to a suitable value so as to produce a desired optical power
from the laser diode whenever an enable signal is applied to the
system via line 450. Because this optical power changes over time
due to aging of the laser diode, the system of FIG. 6 changes the
value of the laser current to compensate for the reduction in
optical power. The operation will be better understood by the
circuit description below.
[0037] The electrical signal applied through line 410 is
capacitively coupled into the laser diode 601. While shown coupled
into the cathode terminal of laser diode 601 it will be understood
that there are several alternative coupling methods by which the
electrical signal can be coupled into laser diode 601. The
electrical signal alters the current flowing through laser diode
601 and consequently the optical signal appearing on link 361
reflects the characteristics of the electrical signal in the
optical domain. The laser diode current on line 422 is also
controlled by the transistor driver circuit comprising transistor
603, which is selected to have a suitable bandwidth of operation.
Transistor 603 is illustrated in FIG. 6 as an npn device, but it
will be understood that other devices can be used alternatively.
Such alternative devices include pnp transistors,
field-effect-transistors, and op-amp drivers. The
collector-to-emitter current of transistor 603 is the same as the
quiescent current flowing through laser diode 601. The amplitude of
the collector-to-emitter current is determined by the
base-to-emitter voltage that is applied by op-amp 604 via its
output terminal 667, into transistor 603. Resistors 624, 622, and
623 are DC-biasing elements, and are selected to limit the base
current, and to limit the collector-to-emitter current
respectively. The collector-to-emitter current is set below the
allowable maximum current through the laser diode 601, so as to
avoid potential damage to the diode from excessive current flow.
Resistors 622 and 623 also determine transistor driver gain, which
directly affects loop parameters.
[0038] Op-amp 604 is a part of an integrator 629 that includes
several resistor and capacitors, chosen appropriately to provide
integrator functionality. The integrator time constant is largely
determined by the values selected for resistor 605 and capacitor
606. The integrator time constant is generally selected to provide
high gain for low-frequency signals that may be present in a
feedback voltage that is present at feedback voltage node 614,
which is connected to input terminal 615. The values for resistor
605 and capacitor 606 are selected to obtain a desirable damping
factor, and furthermore, are selected such that the open loop
frequency response of the integrator 629 in conjunction with a
transimpedance circuit 628 comprising op-amp 610, reaches unity
gain below the lowest modulating frequency present in the loop 652.
Resistor 607 operates as a damping resistor, and is selected to
improve the dynamic response of the loop 652. The dynamic response
of the loop 652 includes fast settling time as well as a desirable
transient response. Resistor 608 and capacitor 609 are typically
selected to provide appropriate differential balance between input
terminals 615 and 616 of op-amp 604.
[0039] Op-amp 610 and associated components comprise a
transimpedance amplifier circuit 628 that converts the laser
monitor current into a feedback control voltage that is provided to
feedback voltage node 614. Resistor 611 is selected to be
sufficiently small in value so that the pole caused by resistor 611
in conjunction with the input capacitance of the negative input
terminal of op-amp 610 is high enough in frequency to avoid
negative impact upon the loop phase margin. The loop phase margin
is dominated by the pole contributed by the combination of resistor
611 in parallel with capacitor 612. Capacitor 612 is selected such
that the transimpedance amplifier 610 provides attenuation above
the lowest modulation frequency. The gain of the transimpedance
amplifier 610 is set to compensate for a low value of resistance in
the potentiometer 651, or alternatively, the gain is selected to
permit a setting of a desired value of resistance in the
potentiometer 651. The R-C time constant of the potentiometer 651
together with the intrinsic package capacitance of the laser
transmitter 420 determines in part, the transient response of the
loop 652, and if the potentiometer is selected improperly, can lead
to excessive ringing and overshoot in the feedback voltage.
[0040] In this exemplary embodiment, zener diode 615 together with
resistor 626 comprises a reference voltage source 627. In other
embodiments, other circuits that generate a reference voltage may
be used. Persons of ordinary skill in the art will recognize that
there are many alternative circuits that can be used to generate a
reference voltage. The components of the reference voltage source
627 are typically selected to have certain desirable operating
characteristics over temperature and other environmental
conditions. Input selector switch 617 is configured to select the
reference voltage source 627 when the digital enable signal on line
450 is asserted, and alternatively, to select the ground node 616
when the digital enable signal on line 450 is de-asserted. While
many alternative switches may be used, a solid state switch is used
where fast switching times are desirable.
[0041] In one embodiment, among many, the zener diode 615 is a 2.5V
zener, and input selector switch 617 provides this 2.5V to the
first input terminal of the integrator 629, specifically, the
positive input terminal of op-amp 604 in this example. When the
reference voltage source is provided to the integrator 629, the
laser diode current in line 422 begins to flow, thereby turning on
the laser diode 601 and producing an output optical signal into
link 361. When the laser diode current is flowing in line 422, the
laser diode monitor current in line 421, which is proportional to
the laser diode current, also begins to flow. The voltage drop
across the potentiometer 651 is then translated by transimpedance
amplifier 628 to generate a feedback voltage of 2.5V at feedback
voltage node 614, and the feedback loop 652 stabilizes with a
certain amount of time, and with certain timing
characteristics.
[0042] When the input selector switch 617 operates to select ground
node 616, the output voltage of integrator 629 tends to zero,
thereby shutting off the laser diode current in line 422 and the
output optical signal on link 361. The operation of input selector
switch 617 places the feedback circuit in the burst operative mode,
because the enable logic on line 450 is digital in nature, and the
output optical signal is turned on and turned off in a digital
mode. Zener diode 615 is selected to provide a zener voltage that
lies in a range between the voltage rails of op-amp 604. Zener
diode 615 is also selected to have good temperature stability. The
output power is set by operating potentiometer 651. For example, if
the potentiometer is set to cause a first laser diode monitor
current to flow in line 421, the feedback loop 652 operates to
cause the laser diode current on line 422 to follow suit such that
the monitor current can be maintained at the selected value. The
laser diode current directly determines the power of the output
optical signal.
[0043] Attention is now drawn to FIG. 7, which is a flow chart of a
method of operating an analog laser transmitter. In step 705, an
analog signal is provided to the analog laser transmitter. In one
exemplary embodiment, the analog laser transmitter is an
analog-modulated CATV reverse transmitter of a PON system, and the
analog signal is an upstream signal of the PON system. In step 710,
a digital enable signal is provided to a controller circuit that is
coupled to the analog laser transmitter. In one exemplary
embodiment, the digital enable signal is a TDMA control signal of
the PON system. In step 715, the analog laser transmitter is turned
on when the enable signal is asserted, and alternatively, turned
off when the enable signal is de-asserted. The turn on and turn off
characteristics have been described above with reference to other
figures, and will not be repeated here in the interests of
brevity.
[0044] In other alternative embodiments, the method includes one or
more steps such as providing a reference voltage, providing a
ground node, providing an input selector switch, activating the
input selector switch using the TDMA control signal to select the
reference voltage, whereby the analog-modulated CATV reverse
transmitter is turned on, and activating the input selector switch
using the TDMA control signal to select the ground node, whereby
the analog-modulated CATV reverse transmitter is turned off.
[0045] It should be emphasized that the above-described embodiments
of the present disclosure, particularly, any "preferred
embodiments" are merely possible examples of implementations,
merely setting forth a clear understanding of the principles of the
disclosure. Many variations and modifications may be made to the
above-described embodiments of the disclosure without departing
substantially from the spirit of the principles of the disclosure.
All such modifications and variations are intended to be included
herein within the scope of the disclosure and present disclosure
and protected by the following claims.
* * * * *